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    Re: sight reduction with GPS receiver
    From: Herbert Prinz
    Date: 2005 Mar 21, 13:08 -0500

    Hello Frank,
    
    > Near as I can tell, you're saying that new (or new-ish) GPS receivers
    > calculate geodesics on the Earth as shortest distance arcs on the
    > ellipsoid.
    
    That's exactly what I am saying.
    
    > is this so? I would think that it would depend very much on the
    > software in the device. Considering the relatively small importance of
    > this issue in practical circumstances, it would not surprise me a bit
    > if many GPS devices calculated distances as spherical great circle
    > distances.
    
    Reflecting the economic reality of modern s/w production for the end
    user market, the small importance of the issue is reflected in the want
    of documentation rather than quality of implementation. The manuals that
    come with my various Garmins just speak of "distance", never ever
    specifying what kind. Simple tests with models 45, 12, 12XL reveal that
    they all display distance along the geodesic line on the chosen
    reference ellipsoid For consistency, I would expect no less from a unit
    promising 15m positioning accuracy.
    
    To achieve the 4 digit accuracy on a Garmin display, an approximative
    formula is sufficient. Even a full numerical integration would not be a
    big deal for an infrequent computation such as this. I see no reason not
    to do it right.
    
    Paul Hirose obviously has a unit that displays GCD. I would like to hear
    which model it is and whether it does this automatically or has to be
    coerced into doing it. I have a faint memory of my first Magellan (as
    big as a shoe box) being able to display GCD. I may be wrong. It has
    been many years since it died of corrosion, so I am unable to check. On
    most consumer products it is impossible to specify a spherical earth: If
    the so called "user datum" feature is present at all, often the
    flattening has to be entered as its reciprocal value. In other cases,
    only small corrections to the WGS84 parameters are allowed. In my
    opinion, a reasonable self defense measure of the industry against
    improper use.
    
    Herbert Prinz
    
    
    

       
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